1. Field of the Invention
This invention relates generally to magnetic head servo control systems and, more particularly, to disk drive position control systems that determine the location of a head relative to disk tracks.
2. Description of the Related Art
In a conventional computer disk drive sector servo system, servo information is stored in servo bursts recorded in a magnetic storage material as a series of magnetic flux reversals. When the disk rotates beneath a read/write head, a magnetic read element of the head senses the changes in flux and produces a varying electrical readback signal. The electrical signal can be decoded to indicate the head position relative to tracks of the disk. In this way, the read/write head can be accurately positioned relative to data tracks of the disk for data read and write operations.
Each disk surface of a sector servo disk includes concentric or spiral tracks that are divided into sectors having a short servo track information area followed by a customer data area. The servo track information area typically includes a sector mark, track identification data, and a servo burst pattern. The sector mark indicates that servo information immediately follows in the track.
FIG. 1 shows a conventional disk drive system 20 having a rotatable storage disk 22 and a rotary arm 24 that is moved by a servo motor 26. The read/write head 28 is suspended over the disk at one end of the arm. The disk 22 has concentric tracks 30 and is divided into sectors that are defined by circumferentially spaced sector mark fields 32, of which two are shown. It should be understood that conventional disk drives typically contain approximately one hundred sectors per track and more than 5000 data tracks; fewer are indicated in FIG. 1 for simplicity. Customer data is recorded by a user into the track spaces 33 between the sector marks. The read/write head 28 produces a readback signal when reading information from the disk 22 and receives a write signal when recording information onto the disk surface. The readback signal and write signal are carried to and from the head 28 over a data cable 34, which is coupled to a disk drive controller 36.
When the read/write head 28 is over servo information recorded into the disk, the disk controller 36 receives position information and in response generates a position error sensing (PES) signal that indicates the position of the head relative to a disk track. The PES signal is used by the disk drive controller to generate servo commands that control the servo motor 26 and are provided over a servo line 38 to maintain the head in a position centered above one of the tracks.
FIG. 2 shows the read/write head 28 of FIG. 1 in greater detail, shown in an exploded view providing better visualization of the component. The head 28 comprises what is commonly referred to as a magneto-resistive (M-R) head, which includes an M-R read element 40 and an inductive write element 42. The M-R read element 40 is placed on a non-magnetic gap material 44 located on a magnetic shield piece 45. The write element of the head includes a magnetic gap 46 containing a magnetic pole piece and electromagnetic coils (not shown in FIG. 2). A second non-magnetic gap material 47 is placed over the M-R read element 40 and leads. The write element is placed on the second gap material 47, which is over the M-R read element.
Two electrical wires 48, 50 are connected to read contacts 52, 54 respectively, and carry the sensed readback signal from the M-R read element to signal processing circuitry 56. The combined read/write head shown in FIG. 2 permits a single head assembly to include both read and write elements and thereby simplifies production and design.
The disk controller 36 controls the servo motor 26 (FIG. 1) to maintain the read/write head 28 above a magnetic track 60 of the disk 22 in response to the head readback signal. As noted above, the head readback signal is generated from sensed servo pattern bursts recorded in the disk track 60. The servo pattern bursts are recorded in the disk tracks as magnetic field transitions that extend across the width of the disk tracks.
FIG. 3 shows a conventional servo burst pattern comprising an A, B, C, D quadrature burst pattern that is repeated for each servo sector. The bursts are part of the information following the sector mark (FIG. 1). Each burst of the quad-burst to servo pattern shown in FIG. 3 is typically made up of two parts, each being one-half of a data track pitch (DTP), as indicated by the data track numbers along the left side of the drawing showing respective data track centerlines. It should be understood that other servo pattern widths are possible. For example, many conventional disk drive systems utilize servo patterns that are two-thirds the width of a data track. In addition to quad-burst servo patterns, it is also common to use dual burst patterns, which generally comprise only the A and C servo bursts of the quad-burst pattern illustrated in FIG. 3.
The servo pattern bursts A, B, C, and D are produced by energizing write coils in the read/write head 28 during a servo writing operation before final disk drive assembly. When the write coils of the read/write head are energized, they do not record flux transitions that correspond exactly to the actual width of the track. To the contrary, the flux transitions typically span about 60% to 90% of the track pitch, depending on the tolerance of the width of the write element and the recorded density. The write coils cannot record a full-width pattern because the width of the write element is less than the data track pitch for data handling purposes. Therefore, to get a servo pattern with bursts that span substantially the full width of a track, it is necessary to make multiple passes.
The servo pattern bursts are typically produced with a two-step servo write process, as shown in FIG. 3. The process steps are generally referred to as "move and write" because the read/write head is moved, a portion of the servo burst is written to the disk, and the process is repeated. When the read/write head is moved, it is moved radially a predetermined distance that is typically one-half DTP. After the second move and write step, the second portion of the servo pattern burst is recorded.
Because the magnetic flux transitions written by the write element into the disk are usually greater than one-half DTP, the total width of the A burst is now greater than one data track width (DTW), proper size is obtained on the third pass by erasing part of the burst at the next half DTP position, which is commonly called trimming. Thus, with each pass that writes part of the servo pattern, part of the previously written flux transitions are erased or written over. It should be appreciated that the two burst halves making up the servo pattern burst must be aligned radially (that is, the flux transitions must be oriented along the same radial line from the disk center) so the transitions are in phase. The alignment requirement restricts the servo frequency to be significantly lower than the data frequency, which reduces the servo signal-to-noise ratio.
Typically, the read/write head that will be used in reading and writing customer data after the disk has been sold is the same head as the read/write head used in producing the servo pattern bursts. Accordingly, the read/write head is generally optimized for reading and writing customer data, not servo patterns. As a result, the read/write head is generally somewhat more narrow that the DTP to allow for write element size tolerance and servo track following imperfections. Typically, the write element of a read/write head is approximately 85% of DTP and the effective read width of the M-R read element is approximately 50% of DTP. Using a narrow read element causes non-linearity in the response of the read/write head when reading the servo information. The non-linearity can be reduced somewhat by using narrow servo write steps; hence, many systems today use servo write steps that are one-third DTP, so that resulting servo pattern bursts are two-thirds DTP.
Narrow servo write steps mean more steps in the servo writing process. The number of servo writing steps for a five thousand data track disk, for example, increases from 10,000 steps to 15,000 steps if the servo pattern bursts are made more narrow by using one-third servo write steps in place of one-half servo write steps. Thus, there is potentially a 50% increase in servo writing time as a result of increasing the number of servo passes to achieve better servo linearity and thus more accurate tracking. If the improved accuracy permitted 5% more tracks to be fit into the same recording band, then there would need to be 1575 servo passes. Thus, a 5% capacity increase would need 57.5% more servo writing time.
From the discussion above, it should be apparent that there is a need for a disk drive with a servo pattern that can be written with a reduced number of servo write steps and that provides increased linearity in the readback signal. The present invention fulfills this need.